The manipulation and storage of fluids plays an important role in fields such as chemistry, microbiology and biochemistry. These fluids may include liquids or gases and may provide reagents, solvents, reactants, or rinses to chemical or biological processes. While various microfluidic methods and devices, such as microfluidic assays, can provide inexpensive, sensitive and accurate analytical platforms, the handling and storage of fluids—such as sample introduction, introduction of reagents, storage of reagents, separation of fluids, modulation of flow rate, collection of waste, extraction of fluids for off-device analysis, and transfer of fluids from one device to the next—can add a level of cost and sophistication. Accordingly, advances in the field that could reduce costs, simplify use, and/or improve fluid manipulation and storage in microfluidic systems would be beneficial.